# Conventions

- World frame: Z-up, metres, levelled per Nokov capture block; z = 0 at the Nokov origin.
  Per-session details in `meta.json` under `canonical`.
- Time: seconds, t = 0 at the first solved Body0 frame. Every stream keeps its own `t`; nothing is resampled.
- Quaternions: xyzw, global orientation in the world frame.
- Poses: 4x4 homogeneous `T_world_<x>` maps points from frame <x> into the world frame.
- `valid` masks mark frames with a usable value; invalid frames may hold NaN.
- Merged npz files prefix keys by source: `left_`, `right_`, `rig_`, `tracker_`, or the video name
  (`head_left_t`, `wrist_right_frame_index`, ...).
- Objects: `motions/objects.npz` (all object instances on the body grid), see `objects.md`.
- Hands: `motions/smplx.npz` carries finger articulation from GSI hand tracking on the mocap body wrist
  (`hand_observed`, `hand_quality_deg`). `motions/hands_mano.npz` is the hand tracking alone at 30 Hz: native MANO
  per side (flat_hand_mean=True, axis-angle), 21 joints in the world and in the left_front OpenCV camera frame, placed
  with the mocap head pose; `usable` and `body_consistent` (wrist within 100 mm / 30 deg of SMPL-X) masks. Its
  `convention` key has the details.
- SLAM: `cameras/slam.npz` is GSI's visual-inertial SLAM of the left_front camera as delivered (`T_slam_cam`) and
  rigidly aligned to the world (`T_world_slam`, `T_world_cam`), with ATE against the mocap head in `metrics_json`.
- `raw/` holds vendor files unchanged: Nokov exports (bvh/csv/trc, millimetres, Y-up) and device sqlite (.db).